Telomeres are specialized structures at the ends of linear chromosomes that protect them from enzymatic attack and preserve chromosome stability. During the growth of human cells the telomeres shorten with every division until they reach a critical length. Such short telomeres become dysfunctional, lose their protective features, and the cells recognize them and respond with permanent arrest or death. Consequently, replication associated telomere-shortening represents a powerful tumor suppressive mechanism, evidenced by the fact that almost all human tumors find a way to maintain their telomeres at a constant length. Tumor cells activate one of two potential pathways to counteract telomere shortening. 90% of cancers up-regulate the telomerase complex, capable of elongating telomeres by reverse transcription. 10% of tumors activate a recombination-based mechanism termed ALT. While fewer tumors initially rely on ALT, it can be considered the dominant mechanism, since ALT can be initiated upon inhibition of telomerase, rendering targeting of telomerase as cancer therapy without effect. While models such as yeast, mice and human cells have emphasized the importance of telomeres for cellular and organismal survival, they have so far failed to provide detailed information about regulation of both, telomerase and ALT. Furthermore, no multicellular system for studying the role of ALT in aging and cancer is available. Here I propose to take advantage of our finding that C. elegans telomeres represent primitive telomeres that rely on and regulate ALT as well as telomerase. I hypothesize that nematode telomeric proteins are regulators of these pathways, and that we can take advantage of them to understand the roles of ALT in organismal survival and tumorigenesis. AIM 1 is designed to isolate the complexes that regulate telomerase and ALT, to characterize the individual proteins in the complex, and to analyze their telomeric functions in detail. AIM 2 is designed to investigate the generation and maintenance of telomeric C and G overhangs in nematodes and mammals in detail, based on the hypothesis that C overhangs are defining features of ALT cells. In AIM 3 we will study how ALT can replace telomerase as telomere maintenance mechanism in a multicellular system. We have generated such an organism;we will characterize it and use it as a screening tool for regulators of the ALT pathway. In summary, this proposal suggests to establish C. elegans as a model system for telomere maintenance in cancer, and to translate between findings in nematodes and cancer cells. PUBLIC HEALTH RELEVANCE: Cancer is an extremely variable disease, and the multiple different tumor types share few common aspects that can be targeted for therapy. Given that all cancers need to find a way to maintain telomere length in order to obtain immortality, telomeres, the natural ends of linear chromosomes, represent one of these universal targets. This proposal consists of three specific aims designed to understand telomere length maintenance mechanisms in the more primitive regulative system of the nematode C elegans, which can be decoded more efficiently, with the final goal of the development of inhibitors of these pathways as cancer therapy tools.